1. Field of the Invention
This invention is related to the general field of radiation imaging for medical applications. In particular, the invention provides a new method and apparatus for producing a diagnostic image of the portion of the body affected by a tumor, so that the required dosage of radiation can be accurately delivered to the prescribed target volume.
2. Description of the Prior Art
The main object of radiotherapy is to deliver the prescribed dosage of radiation to a tumor in a patient while minimizing the damage to surrounding, healthy, tissue. Since very high energy radiation (produced at 4 to 25 million volts, typically generated by a linear accelerator) is normally used to destroy tumors in radiotherapy, the high energy is also destructive to the normal tissue surrounding the tumor. Therefore, it is essential that the delivery of radiation be limited precisely to the prescribed target volume (i.e., the tumor plus adequate margins), which is accomplished by placing appropriately constructed shielding blocks in the path of the radiation beam. Thus, the goal is to accurately identify the malignancy within the body of the patient and to target the prescribed dosage of radiation to the desired region on the immobilized patient.
To that end, the ideal procedure requires the identification of the exact anatomical location of the tumor and the corresponding accurate positioning of the radiation field during treatment. This could be easily achieved if it were possible to locate and treat the tumor at the same time. In practice, though, this is not possible because the equipment used to identify the tumor (x-ray machine, computed tomography equipment, or the like) is separate from the equipment used for the therapeutical irradiation of the patient, requiring the movement and repositioning of the patient from one piece of equipment to the other. As illustrated in schematic form in FIG. 1, a conventional treatment unit 10 consists of a linear accelerator (linac) head 2 mounted on a gantry 4 so that its collimated high-energy emissions HR irradiate a patient P lying on a gurney 6 directly below through shielding blocks 8 attached to the head. A bracket 12 supporting a detector 14 may be mounted on the opposite side of the head within the field of radiation in order to take radiographs of the patient being treated. The gantry 4 is movable around a pivot 16 to permit the rotation of the head (and of the detector) around the patient to afford different views of the area to be treated ("multiple fields" treatment). The normal procedure involves the use of a diagnostic simulator, which is a diagnostic x-ray machine with the same physical characteristics of the radiation therapy machine (schematically also represented by FIG. 1, where a diagnostic x-ray head replaces the linac head 2), so that the field of view of the low-energy x rays emitted in the simulator is the same as that of the high-energy radiation emitted in the radiation therapy machine. Prior to treatment, the patient is radiographed using the simulator and an image of the target area is obtained with low-energy radiation (in the order of 100 kVp), which yields good image quality. The exact target volume is then delineated on the radiograph by a physician and matching shielding blocks are constructed to limit the field of view of the irradiating machine to the region so delineated. A mold of the shielding blocks is first cut out of plastic material (normally polystyrene) with a shielding-block cutter, a machine that reproduces exactly the relative positions of the linac head, the shielding blocks and the detector as they stand in the treatment unit. By using mechanical means, the shielding blocks are cut so that the field of irradiation from the treatment unit will corresponds exactly to the area delineated by the physician on the diagnostic radiograph. The final shielding blocks are then made from the mold with lead alloys that attenuate considerably the propagation of radiation. Thus, the shielding blocks function as templets that limit the radiation treatment to the areas left open within the contour of the shielding blocks. In addition, it is common practice to mark the skin of a patient with reference markings that are used in aligning the position of the patient with the field of emission of the radiation therapy machine.
These apparently sound procedures in fact suffer from serious practical shortcomings. Errors in positioning the shielding blocks between the radiating source and the patient, as well as incorrect beam alignment and patient movement, all have a cumulative effect reducing the accuracy of the procedure. Even the markings on the skin of the patient may be the cause of alignment problems because of shifting of the skin with respect to the patient's internal anatomy as a result of body motion or, over a period of time, even of body changes. Thus, the area actually irradiated during the therapeutic session often does not correspond to the area delineated in the radiograph generated by the simulator.
Positioning errors during irradiation have been found to have very serious consequences for the successful prognosis of the treatment. For example, researchers have been able to correlate the recurrence of lymphoma to such positioning errors ( J. E. Marks, A. G. Haus, H. G. Sutton and M. L. Griem, "Localization Error in the Radiotherapy of Hodgkin's Disease and Malignant Lymphoma with Extended Mantle Fields," Cancer 34, 83-90, 1974); and it has been found that improved tumor control of nasopharingeal carcinomas can be related to greater accuracy in the delivery of calculated dosages of radiation (J. E. Marks, J. M. Bedwinek, F. Lee, J. A. Purdy and C. A. Perez, "Dose-Response Analysis for Nasopharyngeal Carcinoma: An Historical Perspective," Cancer 50, 1042-1050, 1982). Similarly, it has been found that shielding inaccuracies have resulted in significantly lower primary tumor control and survival of patients of oat cell lung cancer (J. E. White, T. Chen, J. McCracken, P. Kennedy, H. G. Seydel, G. Hartman, J. Mira, M. Khan, F. Y. Durrance and 0. Skinner, "The Influence of Radiation Therapy Quality Control on Survival, Response and Sites of Relapse in Oat Cell Carcinoma of the Lung," Cancer 50, 1084-1090, 1982); and that the local recurrence of Hodgkin's disease was significantly higher when the radiation field did not adequately cover the tumor (J. J. Kinzie, G. E. Hanks, C. J. Maclean and S. Kramer, "Patterns of Care Study: Hodgkin's Disease Relapse Rates and Adequacy of Portals," Cancer 52, 2223-2226, 1983).
The only technique widely used today to check the accuracy of the radiation field is by imaging with the radiotherapy beam itself at the time of treatment. Prior to treatment, a "portal" image is obtained by using the therapy beam (at high energy) and the resulting exposure is visually compared with that taken with the simulator (at low energy). This technique is therefore known as "portal imaging" or "therapy verification," and is repeated periodically during the period of radiation treatment. Unfortunately, though, because of the high-energy radiation emitted by the treatment beam (produced at 4-25 million volts), the resulting portal images have poor resolution and show very poor contrast between soft tissues and bones, often making the images totally unsuited for verification by comparison with the low-energy images produced by the simulator. See, for example, R. T. Droege and B. J. Bjarngard, "Influence of Metal Screens on Contrast in Megavoltage X-Ray Imaging," Med. Phys. 6, 487-492, 1979; L. E. Reinstein, M. Durham, M. Tefft, A. Yu and A. S. Glicksman, "Portal Film Quality: A Multiple Institutional Study," Med Phys. 11, 555-557, 1984; W. R. Lutz and B. E. Bjarngard, "A Test Object for Evaluation of Portal Film," Int. J. Radiat. Oncol. Biol. Phys. 11, 631-634, 1985; and P. Munro, J. A. Rawlinson and A. Fenster, "Therapy Imaging: A Signal-to-Noise Analysis of Metal Plate/Film Detectors," Med. Phys. 14, 975-984, 1987. Indeed, positioning errors occur very frequently in spite of the use of portal images. See J. E. Marks, A. G. Haus, H. G. Sutton and M. L. Griem, "The Value of Frequent Treatment Verification Films in Reducing Localization Error in the Irradiation of Complex Fields," Cancer 37, 2755-2761, 1976; R. W. Byhardt, J. D. Cox, A. Hornburgh and G. Liermann, "Weekly Localization Films and Detection of Field Placement Errors," Int. J. Radiat. Oncol. Biol. Phys. 4, 881-887, 1978; Huizenga, P. C. Lenendag, P. M. Z. R. De Porre and A. G. Visser, "Accuracy in Radiation Field Alignment in Head and Neck Cancer: A Prospective Study," Rad. Oncol. 11, 181-187, 1988; R. G. Pearcy and S. E. Griffiths, "The Impact of Treatment Errors on Post-Operative Radiotherapy for Testicular Tumors," Br. J. Radiol. 58, 1003-1005, 1985; I. Rabinowitz, J. Broomberg, M. Goitein, K. McCarthy and J. Leong, "Accuracy of Radiation Field Alignment in Clinical Practice," Int. J. Radiat. Oncol. Biol. Phys. 11, 1857-1867, 1985; and W. C. Lam, M. Partowmah, D. J. Lee, M. D. Wharam and K. S. Lam, "On-Line Measurement of Field Placement Errors in External Beam Radiotherapy," Br. J. Radiol. 60, 361-367, 1987. Imaging devices other than X-ray film have been used in an attempt to improve the quality of the image produced during therapy verification. These include metal and fluorescent screens in contact with conventional film, and non-film imaging processes and devices such as xeroradiography, liquid ionization chambers, fluoroscopic imaging, linear diode arrays, photostimulable phosphors, and others. In addition, various image processing techniques (both analog and digital) have been used to enhance the quality of the final verification image; but all these methods and devices have resulted only in a limited success in yielding a good quality, and therefore useful, diagnostic image. Real-time portal imaging using video techniques has also been proposed, so that patient movement can be monitored during treatment. Because they all use the high-energy therapy beam as the source of radiation, though, the quality of the image remains poor. See N. A. Baily, R. A. Horn and T. D. Kampp, "Fluoroscopic Visualization of Megavoltage Therapeutic X-Ray Beams," Int. J. Radiat. Oncol. Biol. Phys.6, 935-939, 1980; M. V. Herk and H. Meertens, "A Digital Imaging System for Portal Verification," in "The Use of Computers in Radiation Therapy," I. Brunvis Ed., North Holland, 371-373, 1987; S. Shalev, T. Lee, K. Leszczynski, S. Cosby and T. Chu, "Video Techniques for On-Line Portal Imaging," Comp. Med. Imag. Graph. 13, 217-226, 1989; and P. Munro, J. A. Rawlinson and A. Fenster, "A Digital Fluoroscopic Imaging Device for Radiotherapy Localization," Int. J. Radiat. Oncol. Biol. Phys. 18, 641-649, 1990. A survey of 23 different radiotherapy departments shows that at each of eight institutions (i.e., 35 percent of the 23 institutions sampled) more than 3/4 of the submitted portals were evaluated as poor in quality. Furthermore, it shows that approximately one-half of the institutions were producing poor-quality films at a rate of at least 50 percent. See Reinstein, L. E., M. Durham, M. Tefft, A. Yu, and A. S. Glicksman, "Portal Film Quality: A Multiple Institutional Study," Med. Phys. 11, 555-557, 1984.
Another, logical, approach to obtaining diagnostic quality portal films has been by mounting an x-ray tube on the head of the treatment unit as close to the linac gantry as possible. See P. J. Biggs, M. Goitein and M. D. Russell, "A Diagnostic X-Ray Field Verification Device for a 10 MV Accelerator," Int. J. Radiat. Oncol Biol. Phys. 11, 635-643, 1985. The x-ray tube is aligned with the linac emission field so that, to the extent possible within the physical constraints of both devices, the x-ray emissions have the same field of view of the high-energy radiation. As a result, the image received on a film placed on a detector tray on the opposite side of the patient by exposure to either source of radiation is theoretically almost exactly the same. In order to implement this approach, though, a special shielding-block holder coupled to the gantry has to be made, disabling the normal rotation of the linac's collimator and limiting the adjustment capabilities of the equipment. Thus, the complexity of the procedure, the oblique view of the diagnostic beam and the increased time required for each treatment have prevented this technique from gaining widespread acceptance. Furthermore, this method is unsuited for real-time portal imaging.
A similar approach has been followed by placing an x-ray tube at a fixed angle with respect to the axis of the therapeutic beam, so that the x-ray beam and the therapeutic beam have coinciding isocenters corresponding to the location of the radiation target. By rotating the gantry of the radiation unit by that angle, the target can be irradiated from the same point either with a treatment beam or an x-ray beam, with every other variable remaining unchanged. Therefore, verification can be obtained simply by rotating the gantry and switching from one mode of operation to the other. The main problem with this approach is the inevitable angular error introduced during the rotation of the gantry. In addition, because of the alternative use of either mode of operation, this equipment is also not suitable for real time verification.
Therefore, it would be very desirable to have a simpler and more accurate verification imaging system for radiation therapy verification, especially for real time applications. This invention relates to the use of a conventional x-ray tube and conventional imaging devices in a novel geometric configuration to produce such an improved verification imaging system.